Heretofore, so-called chemical exothermic agents that utilize the exothermic reaction between a chemical substance and water have been used as a heating device to heat heat-and-serve meals such as ekiben (train lunch box).
Conventionally, the reaction due to the addition of water to calcium oxide has been used as a mainstream exothermic agent. The exothermic agent that utilizes the exothermic reaction between calcium oxide and water satisfies the regulations under the Food Sanitation Law. However, the reaction between calcium oxide and water, if it is accompanied by violent heat generation, may produce a temperature as high as 300 to 450° C. and involves difficulty in temperature control, as well as causing a burn risk when one uses it to heat portable lunch boxes such as ekiben.
A chemical exothermic agent that deals with this defect of the exothermic agent which utilizes the exothermic reaction between calcium oxide and water has been proposed and it utilizes the reaction between water and a mixture consisting of calcium oxide and an aluminum powder. In particular, a chemical exothermic agent has been proposed that focuses on the particle size distribution of the aluminum powder.
For instance, claim 1 of Patent Document 1 discloses “an exothermic agent consisting of, based on its weight, 15 to 30% of a calcium oxide powder of 100 mesh (−150 μm≧90%) to 200 mesh (−75 μm≧95%) and 70 to 85% of an aluminum powder having such a particle size distribution that a −330 mesh (−45 μm) is 40 to 60%, a +330 mesh (+45 μm) is 15 to 30%, a +235 mesh (+63 μm) is 15%>, and a +200 mesh (+75 μm) is 10%>.”
Paragraph 0025 of Patent Document 1 suggests that the applicable aluminum powder is one having such a particle size distribution that a −330 mesh (−45 μm) is 35 to 60%, a +330 mesh (+45 μm) is 15 to 30%, a +235 mesh (+63 μm) is 5 to 15%, and a +140 mesh (+106 μm) is 7%> or one having such a particle size distribution that a −330 mesh (−45 μm) is 70 to 90%, a +330 mesh (+45 μm) is 30%>, a +235 mesh (+63 μm) is 3%>, and a +200 mesh (+75 μm) is 2%>.
The defect of the chemical exothermic agent that utilizes the reaction between water and the mixture consisting of calcium oxide and an aluminum powder is that it evolves hydrogen gas. When the exothermic agent comprising the mixture of calcium oxide and aluminum contacts water, calcium hydroxide (Ca(OH)2) is first generated in accordance with the reaction formula (1).CaO+H2O=Ca(OH)2  (1)
On the other hand, the aluminum powder reacts radically with calcium hydroxide to generate calcium aluminate and hydrogen in accordance with the following reaction formula (2).2Al+3Ca(OH)2=3CaO.Al2O3+3H2↑  (2)
In the case of this reaction, the evolution of hydrogen (H2) gas increases in proportion to the amount of aluminum. In other words, 3/2 moles of hydrogen (H2) gas evolves per mole of aluminum.
Therefore, the first point of improvement that should be made in the exothermic agent that utilizes the reaction between water and the mixture consisting of calcium oxide and aluminum as described in Patent Document 1 is that the evolution of hydrogen gas is made as small as possible. This is the most important issue that was desired by users in the course of development of uses of the exothermic agent.
Furthermore, in the course of development of uses of the exothermic agent that utilizes the reaction between water and the mixture consisting of calcium oxide and aluminum, users submitted the following request. The exothermic agent should be economical and have high thermal efficiency in that so-called “small-volume” foods such as nursing bottles, canned or bottled Japanese sake, canned coffee, canned soft drinks, as well as shao mai (steamed dumpling with the dough gathered at the top) and nikuman (Chinese steamed pork bun) that are repackaged into smaller sizes can be heated by that exothermic agent to the desired temperature (at which they can be eaten) in a short period of time.
In those cases, the bottom area is in the range from approximately 12 cm2 to 19 cm2. With such a small area, the weight of the chemical exothermic agent that can be accommodated is no more than about 5 g to 10 g. Therefore, the request of users is that even if a chemical exothermic agent that contains calcium oxide and aluminum in specified proportions weighs as small as 5 g, it should be capable of raising the temperature to the neighborhood of 100° C. to 90° C. quickly after its reaction with water and that, thereafter, the temperature drops at a small enough rate so that it can maintain the temperature in the neighborhood of 80° C. even after the lapse of 600 seconds. What is important here is that in order to prevent the user against an accident such as burn, control should be done to ensure that the maximum temperature will by no means reach 100° C. and the temperature should be maintained in the neighborhood of 80° C. even after the lapse of 600 seconds. In particular, maintenance of temperatures in the neighborhood of 80° C. even after the lapse of 600 seconds is an essential condition to meet since the exothermic agent of the present invention may occasionally be used in cold environments below the freezing point, as exemplified by mountain climbing, fishing, field shelters from large-scale disasters, and heating of fighter rations.
The present inventors first made studies on how to increase the unit amount of heat generated. As already mentioned, the reaction between water and the exothermic agent consisting of the mixture of calcium oxide and the aluminum powder follows the reaction formulas (1) and (2).
In particular, in the reaction formula (2), aluminum reacts radically with the calcium hydroxide produced by the reaction formula (1) so as to generate calcium aluminate and hydrogen.2Al+3Ca(OH)2=3CaO.Al2O3+3H2↑  (2)
In the case of this reaction, the amount of heat generated per unit amount of aluminum is so great that an increase in the amount of aluminum will lead to an increase in the total amount of heat generated. However, the evolution of hydrogen (H2) gas increases with the amount of aluminum, so the method of increasing the amount of aluminum is obviously not preferable from the viewpoint of reducing the evolution of hydrogen.
Therefore, given the exothermic agent consisting of the mixture of calcium oxide and aluminum, it is impossible to satisfy the aforementioned need of users by merely adjusting the amount of calcium oxide and/or aluminum.
Regarding a so-called “water-addition exothermic agent” which generates heat by reaction with water, as exemplified by calcium oxide, a method has been proposed to control its heat generation by adding an inorganic salt and/or an organic salt to it.
For example, Patent Document 2 discloses a method of controlling the heat generation from a water-addition exothermic agent consisting of calcium oxide which is characterized by adding to the water-addition exothermic agent an aqueous solution containing an inorganic salt such as potassium carbonate, sodium carbonate, magnesium chloride, magnesium sulfate, potassium chloride, calcium hydroxide or sodium sulfate, or an organic salt such as sodium benzoate or sodium dodecylsulfate.
As described in various documents, it is known that the reaction between calcium oxide and water, if it is accompanied by a violent heat generation, may produce a temperature as high as 300 to 450° C. and involves difficulty in temperature control. The object of the invention described in Patent Document 2 is to ensure that by addition of the aforementioned inorganic salts and/or organic salts, the maximum temperature of the vapor evolved in the reaction between calcium oxide and water which may become as high as 300 to 450° C. is maintained at 100 to 280° C. for 260 to 480 seconds. Therefore, the invention described in Patent Document 2 also fails to meet the users' need mentioned in the foregoing paragraph 0010.
Patent Document 1: Specification of Japanese Patent No. P3467729
Patent Document 2: Japanese Patent Publication No. P 2003-171658 A